1. Field of the Invention
This invention relates to ester-based, in particular diester and polyol ester-based turbo oils, which exhibit superior antioxidancy and reduced deposit forming tendencies. More particularly, it is related to turbo oils comprising esters of pentaerythritol with fatty acids as base stock, and a sulfur-containing carboxylic acid derivative, used as a dual functional additive providing enhanced oxidation stability and reduced deposit formation.
2. Description of the Related Art
Organic compositions such as mineral oils and lubricating compositions are subject to deterioration by oxidation and in particular are subject to such deterioration at high temperatures in the presence of air. This deterioration often leads to buildup of insoluble deposits which can foul engine parts, deteriorate performance, and increase maintenance. This is particularly the case for lubricating oils used in jet aircraft where wide temperature ranges and extreme operating conditions are likely to be encountered. Proper lubricating of aircraft gas turbines, for example, requires ability to function at bulk oil temperatures as low as -65.degree. F. to as high as 450.degree.-500.degree. F.
Most lubricants contain additives to inhibit their oxidation. For example, U.S. Pat. No. 3,773,665 discloses a lubricant composition containing an antioxidant additive mixture of dioctyl diphenylamine and a substituted naphthylamine. U.S. Pat. Nos. 3,759,996; 3,573,206; 3,492,233, and 3,509,214 disclose various methods of oxidatively coupling alkylated diphenylamines with substituted naphthylamines.
U.S. Pat. No. 4,820,430 discloses the lubricant composition containing a copper salt of a propionic acid derivative or an additive prepared by reacting a suitable thiodipropionic acid derivative with a suitable alcohol or amine-containing compound to impart multifunctional and antioxidant characteristics.
U.S. Pat. No. 4,189,388 discloses synthetic lubricating oil composition having improved oxidation stability comprising pentaerythritol ester base oil and containing a phenyl-naphthylamine, a dialkyldiphenyl amine, a polyhydroxy anthraquinone, a phosphate ester and a thioacid derivative. The thioacid derivatives mentioned are thio diester or diamide such as dilaurylthiodipropionate and N,N'-di(.beta.-undecyl)thiodipropionamide.
U.S. Pat. No. 4,157,971 is directed to a similar lubricating oil composition as described in U.S. Pat. No. 4,189,388 except for the thioacid derivative being replaced by an alkyl thioacid ester. Examples of the alkylthioacid ester include 2-butylthio-isooctyl glycolate, 3-butylthio-isohexyl propionate.
U.S. Pat. No. 4,174,284 discloses liquid hydrocarbon-containing organic composition exhibiting improved anti-oxidation properties in the presence of a hydrocarbylpolythiobenzoic acid. The number of sulfur atoms in a polythio linkage ranges from 2 to 8, and the examples of such compounds cited include 2-n-hexyl-dithiobenzoic acid and 2-n-dodecyltetrathio-4-cyclohexylbenzoic acid.
JP 63,210,194-A discloses thermally and oxidatively stable lube useful as compressor oil, turbo-charger oil, etc. that contains thiodipropionate ester obtained from thiodipropionic acid and tertiary alcohol.
EP 227,948-A discloses a polyolefin stabilizing composition containing a tris-alkyl-phenyl phosphite (I) and a dialkyl-thio-dipropionate (II). II synergistically enhances the stabilizing effectiveness of I to improve the melt-processing and color stability of the polyolefin.
It has now been discovered that the anti-deposition and antioxidant properties of the polyol ester-based turbo oils can be greatly enhanced by the addition of a small amount of a sulfur containing additive, specifically sulfur-containing carboxylic acid derivatives such as thiosalicylic acid (TSA) or thioethers such as thiodipropionic acid (TDPA).